Myotonic dystrophy (DM) is the most common form of muscular dystrophy in adults, affecting approximately 1 in every 8,000 individuals. Myotonic dystrophy type 2 (DM2) is caused by the expansion of the tetra-nucleotide repeat CCTG in the first intron of the ZNF9 gene. The normal biological function of ZNF9 is unknown. Alternative pathogenic mechanisms have been proposed to explain DM2, including loss of function of the mutated gene and an RNA-mediated gain of function in which the transcribed CCUG repeats sequester proteins from their normal functions. We have identified ZNF9 in a novel proteomic screen to identify RNA binding proteins that associate with an internal ribosome entry site (IRES). IRESs mediate translation initiation of specific mRNAs independent of the 5'cap complex. Our functional assays show that ZNF9 is a positive regulator of cap-independent translation initiation. We hypothesize that ZNF9 normally functions as an IRES trans-activating factor (ITAF) for a population of human mRNAs translated by cap-independent mechanisms. The mechanism(s) by which the CCTG expansion in ZNF9 causes DM2 is poorly understood. Recent studies suggest that loss of ZNF9 results in DM2 symptoms. Hence, we plan to investigate the loss-of-function model. The first goal of this proposal is to test whether the CCTG expansion disrupts the normal activity of ZNF9 by measuring cap-independent translation activity in cells from DM2 patients. The answer to this key question is essential for designing a strategy of therapeutic intervention. Our preliminary data strongly show that ZNF9 functions as an initiation factor for cap-independent translation. Human transcripts regulated by ZNF9 are unknown. Therefore, the second goal is to identify human mRNAs that interact with ZNF9. Finally, it is important to know the complete set of proteins that interact with ZNF9 to understand its normal biological function. The interactions of ZNF9 with other proteins are mostly unknown. Thus, the third goal is to identify proteins that associate with ZNF9 using affinity purification of the ZNF9 protein and highly-sensitive mass spectrometry-based proteomics. PUBLIC HEALTH RELEVANCE. Myotonic dystrophy is the most common form of adult muscular dystrophy. The type 2 form of the disease is caused by expansion of CCTG repeats in the non-coding region of the gene ZNF9. This proposal investigates the normal and disease role of ZNF9 in myotonic dystrophy type 2.